This invention relates to a high-voltage ceramic capacitor with improved rupture strength per unit of thickness and compact construction and the method of producing the same.
The recent trend is for electronic products to be made more compact, that is, lighter, thinner, shorter and smaller. It is attended with an increasing demand for smaller capacitors, resistors and the like. In order to meet such a demand, there are being introduced smaller capacitors except high-voltage ones and technically the latter has been left behind in the march of progress.
The capacitance of a capacitor is expressed by EQU C=.epsilon.o.epsilon.S/t
where .epsilon.o=vacuum dielectric constant; .epsilon.=specific dielectric constant; S=area of electrode; and t=space between electrodes.
In the above equation, t is designed in consideration of the voltage applied to the capacitor and the rupture strength per unit of thickness thereof. In the high-voltage capacitor, t is increased to raise the rupture strength, whereas S is increased to obtain desired capacitance, whereby the capacitor becomes large-sized. Consequently, rupture strength per unit of the thickness of a dielectric element must be increased to reduce the size of the high-voltage capacitor. But such is the present state that no significant technical know-how has been ascertained.
FIG. 1 illustrates the construction of a conventional high-voltage ceramic capacitor. In FIG. 1, there is shown a high-voltage ceramic capacitor 1 comprising a ceramic dielectric element 2, electrodes 3, 4, leads 5, 6 connected to the electrodes 3, 4 and a coating 7 prepared from high-insulation synthetic resin such as epoxy resin.
The withstand voltage characteristics of a capacitor of this type are affected by the difference between the external diameter of the ceramic dielectric element 2 and those of the electrodes 3, 4 when the capacitor is designed. When the external diameters of the dielectric and the electrodes are the same, the withstand voltage is maximized, whereas the greater the difference, the lower the withstand voltage becomes.
Any defect within the ceramic dielectric element is also known to be a decisive factor affecting the withstand voltage characteristics. Why the interior defect affects the withstand voltage characteristics will be given as follows:
The known method of preparing the ceramic dielectric element 2 comprises, for instance, adding a binder and water to a dielectric material mainly composed of barium titanate, drying as well as atomizing the mixture using a spray-dryer to make powder, compacting the powder, and firing the mold in an electric furnace to obtain a sintered ceramic dielectric element.
However, because there are produced many pinholes 8 in the mold thus prepared by compacting and pinholes are not completely removed by baking, some of them still remain in the sintered ceramic dielectric element 2. Particularly when the pinholes 8 continuously exist along the direction between the electrodes 3, 4, the disadvantage is that the application of voltage across the electrodes 3, 4 will cause breakdown at a relatively low voltage. Thus the high-voltage capacitor is not available for the above reasons.